Lithography device which uses a source of radiation in the extreme ultraviolet range and multi-layered mirrors with a broad spectral band in this range

ABSTRACT

Lithography device using a source of radiation in the extreme ultraviolet range and multi-layered mirrors with a broad spectral band within this range. 
     Each mirror ( 24, 26, 29 ) includes a stack of layers of a first material and layers of a second material alternating with the first material. The first material has an atomic number greater than that of the second material. The thickness of pairs of adjacent layers is a monotonic function of the depth in the stack. The source ( 22 ) comprises at least one target ( 28 ) which emits the radiation by interAction with a laser beam focused on one of its faces. A part ( 36 ) of the radiation emitted from the other face is used. The invention is applicable to the manufacture of integrated circuits with a high degree of integration.

TECHNOLOGICAL FIELD

This invention relates to a lithography device using a source of extremeultraviolet radiation and multi-layered mirrors provided to reflect thisextreme ultraviolet radiation that is also called “EUV radiation” or“X-UV radiation”.

The wavelength of such radiation is within the range extending from 8 nmto 25 nm.

The invention is applicable most particularly to the manufacture ofintegrated circuits with a very high degree of integration, the use ofEUV radiation enabling one to reduce the etch spacing of such circuits.

STATE OF THE PRIOR ART

In the main, two techniques are known for producing intense EUVradiation. Both of them rely on the collection of photons produced bythe microscopic process of spontaneous emission by a hot plasma of lowdensity which is generated by means of a laser.

The first technique uses a jet of xenon irradiated by a YAG laser, thepower of which is close to 1 kW. In effect, when the nature of the gasand the conditions for expansion into the vacuum are well chosen,clusters are naturally created in the jet through multi-bodyinteractions. These clusters are macro-particles which can contain up toa million atoms and have a density which is sufficiently high (about onetenth of the density of the solid) to absorb the laser beam and therebyheat the atoms of the surrounding gas which can then emit photonsthrough fluorescence.

The second technique uses the corona of a plasma of high atomic number,obtained by interaction of a laser beam, which comes from a KrF laser,the intensity of which is close to 10¹² W/cm², and a solid target ofgreat thickness (at least 20 μm).

The laser beam is focused on one face of this target, called the “frontface” and one uses the EUV radiation emitted by this front face andgenerated by interaction of the laser beam and the material of thetarget.

If the first or the second technique is used, the EUV radiation obtainedcomprises a continuous energy spectrum and with strong emission lines.

The UEV radiation sources which the first and second techniques use havethe following disadvantages.

These sources have an isotropic emission which therefore has a largeangular divergence, and the emitted EUV radiation spectrum includeslines of low spectral width.

It is then necessary to associate with each source, complicated opticalcollection means which enable one to recover the maximum from the wideangular field of emission from the source.

These optical means formed by multi-layered mirrors, must be produced insuch a way that their spectral responses are centered on the emissionline chosen for the exposure of a sample, restricting as much aspossible the loss of intensity due to multiple reflections on themulti-layered mirrors.

A known example of a lithography device using EUV radiation, thewavelengths of which are situated, for example, close to the range from10 nm to 14 nm is diagrammatically shown in FIGS. 1 and 2. Such a deviceis also called an “EUV lithography device”.

This known device is intended to expose a sample E. Generally, this is asemi-conductor substrate 2 (for example made of silicon) onto which alayer of photosensitive resin (“a photo-resist layer”) 3 has beendeposited and it is desired to expose this layer in accordance with aspecified pattern.

After exposure of the layer 3, it is developed and the substrate 2 canthen be etched in accordance with the pattern.

The device in FIGS. 1 and 2 includes

a support 4 for the sample,

a mask 5 comprising the specified pattern in an enlarged form

a source 6 of radiation in the extreme ultraviolet range (FIG. 2),

optical means 7 for the collection and the transmission of the radiationto the mask 5, the latter providing an image of the pattern in enlargedform, and

optical means 8 for reducing this image and projecting the reduced imageonto the layer 3 of photosensitive resin (chosen in such a fashion thatit is sensitive to the incident radiation).

The known source 6 of EUV radiation comprises means of forming a jet Jof clusters of xenon. Only the nozzle 9 which includes these formationmeans is represented in FIG. 2.

The source also comprises a laser (not shown), the beam of which F isfocused onto a point S of the jet J by the optical means of focusing 10.The interaction of this beam F and the xenon clusters generate the EUVradiation R.

The point S is visible in FIG. 1 (but not the nozzle nor the jet ofxenon clusters).

Among the optical means 7 of the device for collection and transmission,there is an optical collector 11 provided with a central opening 12 toallow the focused laser beam F to pass.

This optical collector 11 is positioned facing the jet of xenon clustersand is intended to collect a part of the EUV radiation emitted by thexenon clusters and to transmit this collected radiation 13 toward otheroptical components that also form a part of the optical means 7 forcollection and for transmission.

These optical means 7 for collection and for transmission, the mask 5,which is used in reflection, and the optical means 8 for reduction andfor projection are multi-layered mirrors 14 which selectively reflectthe EUV radiation and are designed in such a way that their spectralresponses are centered on the wavelength chosen for exposure of thelayer of photosensitive resin 3.

It should be made clear that the pattern, in accordance with which onewishes to etch the sample, is formed on the multi-layered mirrorcorresponding to the mask 5, with an enlargement factor suited to theoptical means for reduction and for projection, and this multi-layeredmirror is coated, except for the pattern, with a layer (not shown) whichis capable of absorbing the incident EUV radiation.

Within the wavelength range of EUV radiation, the spectral resolutionΔλ/λ of the mirrors is about 4%.

The breadth of the spectral range usable for exposure is obtained by theconvolution of the spectral breadth of the EUV radiation and thisspectral resolution.

The known multi-layered mirrors to which we will return subsequently andwhich are used in the lithography device shown in FIGS. 1 and 2, have,in particular, the following disadvantage: their spectral band, which iscentered on the wavelength chosen for the exposure, is narrow.

The result is a reduction in the efficiency of the lithography device.

These EUV multi-layered mirrors also have the disadvantage of deformingwhen they are exposed to a high thermal flux coming from the source ofEUV radiation for the device.

DESCRIPTION OF THE INVENTION

One aim of the invention is to propose an EUV lithography device that ismuch more efficient than the known devices considered to be the mosthighly efficient.

The device which is the subject of the invention comprises a source ofEUV radiation which is anisotropic. This EUV radiation is emittedthrough the back face of a solid target of suitable thickness on thefront face of which a laser beam is focused.

Such an anisotropic source enables one to increase the effective portionof the EUV radiation beam and to simplify the collection of thisradiation.

Furthermore, the device which is the subject of the invention comprisesmulti-layered mirrors capable of reflecting the generated EUV radiation,each layered mirror having a spectral band (also called “spectral width”or “bandwidth”) greater than that of the known multi-layered mirrorsmentioned above.

The source used in the invention, the emission spectrum of which iscloser to black body over a broad spectral range, and the multi-layeredmirrors with a broad spectral bandwidth, also used in the invention,work together to lead to a device capable of supplying the sample, whichone wishes to expose with EUV radiation which is more intense than inthe prior art.

Another aim of the invention is to minimize thermal deformation of themulti-layered mirrors which are used in the invention when thesemulti-layered mirrors are exposed to the intense flux of EUV radiation.

To put it precisely, the subject of this invention is a lithographydevice comprising:

a support for a sample intended to be exposed in accordance with aspecified pattern,

a mask comprising the specified pattern in an enlarged form,

a source of radiation in the extreme ultraviolet range,

optical means for the collection and for the transmission of theradiation to the mask, the latter supplying an image of the pattern inenlarged form, and

optical means for the reduction of this image and for the projection ofthe reduced image onto the sample,

the mask, the optical means for collection and transmission and theoptical means for reduction and projection comprising multi-layeredmirrors, each multi-layered mirror comprising a substrate and, on thissubstrate, a stack of layers of a first material and of layers of asecond material which alternate with the layers of the first material,this first material having an atomic number greater than that of thesecond material, the first and second layers co-operating to reflect theextreme ultraviolet radiation, the stack having a free surface ontowhich the radiation to be reflected arrives,

this device being characterized in that the source comprises at leastone solid target, having first and second faces, this target beingcapable of emitting, in an anisotropic way, a part of the extremeultraviolet radiation from the second face of this target, in that theoptical means for collection and for transmission are provided in orderto transmit, to the mask, the part of the extreme ultraviolet radiationcoming from the second face of the target of the source and in that thethickness of pairs of adjacent layers, in the stack of layers that eachmirror comprises, is a monotonic function of the depth in the stack,this depth being counted from the free surface of the stack.

By a “monotonic function” one understands a function which is eitherincreasing or decreasing.

According to a preferred embodiment of the device which is a subject ofthe invention, the target contains a material which is capable ofemitting the extreme ultraviolet radiation by interaction with the laserbeam and the thickness of the target is within a range extending fromabout 0.05 μm to about 5 μm.

Preferably, the target contains a material which is capable of emittingthe extreme ultraviolet radiation through interaction with the laserbeam and which has an atomic number belonging to the group of atomicnumbers ranging from 28 to 92.

According to one particular embodiment of the device which is a subjectof the invention, this device comprises a plurality of targets which aremade integral one with another, the device additionally comprising meansof displacing this plurality of targets so that these targetssuccessively receive the laser beam.

The device may additionally comprise support means to which the targetsare fixed and which are capable of allowing the laser beam to pass inthe direction of these targets, the means of displacement being providedin order to displace these means of support and hence the targets.

These means of support can be capable of absorbing radiation emitted bythe first face of each target which receives the laser beam and ofre-emitting this radiation towards this target.

According to a first particular embodiment of the device which is asubject of the invention, the means of support comprise an openingfacing each target, this opening being defined by two sidewalls,substantially parallel to one another and perpendicular to this target.

According to a second particular embodiment, the means of supportcomprise an opening facing each target, this opening being defined bytwo sidewalls which become further apart from one another as they gotowards the target.

According to a particular embodiment of the invention, the deviceadditionally comprises auxiliary fixed means which are capable ofallowing the laser beam to pass in the direction of the target, ofabsorbing the radiation emitted by the first face of this target and ofre-emitting this radiation towards this target.

According to a preferred embodiment of the invention, the stack whicheach multi-layered mirror comprises, is subdivided into assemblies of atleast one pair of first and second layers and the thickness of theseassemblies is a monotonic function of the depth in the stack, this depthbeing counted from the free surface of the stack.

According to a particular embodiment of the invention, the increases inthickness of these assemblies form an arithmetic progression.

Preferably, the first and second layers of each assembly haveapproximately the same thickness.

By way of example, the first and second layers may be respectivelymolybdenum and beryllium or molybdenum and silicon.

The substrate may, for example, be made of silicon or germanium.

Preferably, the thickness of the substrate is within the range extendingfrom about 5 mm to about 40 mm and the thickness of the stack is about 1μm.

According to a preferred embodiment of the invention, each multi-layeredmirror is fitted with means of cooling this multi-layered mirror inorder to reduce its deformation when it is illuminated by the EUVradiation.

Preferably, these cooling means are provided in order to cool the mirrorto a temperature roughly equal to 100 K.

For example the means of cooling the mirror are liquid helium, Freon,liquid nitrogen or a cooling fluid which is a heat transfer fluid at alow temperature close to 0 K.

The sample that it is desired to expose may comprise a semi-conductorsubstrate on which a layer of photo-sensitive resin is deposited and isintended to be exposed in accordance with the specified pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be better understood on reading the description ofembodiment examples given below, for purely information purposes, beingin no way limitative and which refer to the appended drawings in which:

FIGS. 1 and 2 diagrammatically illustrate a known EUV lithography deviceand have already been described,

FIG. 3 is a diagrammatic view of a particular embodiment of thelithography device which is a subject of the invention,

FIG. 4 is a diagrammatic perspective view of a strip forming an assemblyof targets which can be used in the invention,

FIGS. 5 and 6 are diagrammatic and partial perspective views of sourcesof EUV radiation that can be used in the invention,

FIG. 7 is a diagrammatic and partial perspective view of another sourceof EUV radiation that can be used in the invention,

FIG. 8 is a diagrammatic section view of a known multi-layered mirror,

FIG. 9 shows the curve representative of variations in the reflectorpower as a function of the energy for this known multi-layered mirror(curve I) and for a multi-layered mirror that can be used in theinvention (curve II),

FIG. 10 is a diagrammatic section view of a particular embodiment of amulti-layered mirror that can be used in the invention,

FIG. 11 diagrammatically illustrates the general curvature undergone bya multi-layered mirror when subjected to a high thermal flux,

FIG. 12 diagrammatically illustrates a localized deformation undergoneby a multi-layered mirror when subjected to a high thermal flux,

FIG. 13 shows the curve representing variations in the thermalconductivity k (curve I) and the coefficient of thermal expansion α(curve II), for silicon, as a function of temperature,

FIG. 14 shows the curve representing variations in the ratio α/k as afunction of temperature, and

FIG. 15 is a diagrammatic view of means of cooling a multi-layeredmirror, which can be used in the invention.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

A plasma created by interaction of a solid target and a laser beamcomprises several zones. There is of course the interaction zone that iscalled the corona but there are also, in a successive and simplifiedfashion:

a zone called the conduction zone where the laser beam does notpenetrate, the development of which is controlled by thermal, electronicand radiation conduction, a part of the photons emitted by the ions fromthe corona being emitted in the direction of the cold and dense part ofthe target, and

the absorption and re-emission zone where the photons with high energy,which arrive from the corona or from the conduction zone, are absorbedby the dense and cold material and thereby contribute to the heating ofthis material and therefore to the emission of photons of lower energy.

These photons form a radiation wave which has, in the medium, a favoreddirection of propagation, along the temperature gradient and which can,when the target is not too thick, leave the target through its backface, the face which is geometrically opposite to that where the laserhas interacted. The efficiency of conversion in the back face (the ratiobetween the radiation energy, including all wavelengths, to the energyof the incident laser) can be close to 30%.

This emission from the back face of the target is characterized by aspectral distribution that is very different from that from the frontface since the temperature and density conditions of the zonesresponsible for the emission of photons are very different. Theradiation emitted has a natural angular distribution, even with aperfectly flat target: this radiation is not isotropic.

Furthermore, the characteristic speed of expansion from the back face islower than that from the front face by several orders of magnitude, themajority of the energy being in the form of radiation.

Hence, in the present invention, EUV radiation is used which is emittedthrough the back face of a solid target of suitable thickness, on thefront face of which the laser beam is focused. In this way, anisotropicradiation is obtained and material debris is reduced to a minimum.

To generate the EUV radiation, the target preferably contains a materialwhose atomic number Z is such that 28≦Z≦92.

One can mix or associate with this material other materials that arealso capable of generating EUV radiation having good spectralcharacteristics through interaction with the laser beam.

Apart from this, one or more other materials intended to filterparasitic radiation may also be associated with it.

The thickness of the target, containing the material generating the EUVradiation or active element, is preferably between 0.05 μm and 5 μm.

Preferably, the target is optimized in order to provide an efficientemission through the back face, without the expansion of the materialbeing too great.

The laser characteristics (in particular the duration and the temporalshape of the light pulses that it supplies, the wavelength and theintensity) are also matched to obtaining the thermodynamic conditionsrequired in the target in order to provide optimum EUV conversion in theback face within the desired range of wavelengths which extends, forexample, from 10 nanometers to 20 nanometers.

A particular embodiment of the lithography device that is a subject ofthe invention is represented diagrammatically in FIG. 3.

This lithography device comprises a support 16 for a semiconductorsubstrate 18, for example, a silicon substrate, onto which a layer 20 ofphoto-sensitive resin is deposited, which is intended to be exposed inaccordance with a specified pattern.

Apart from a source 22 of EUV radiation, the device comprises:

a mask 24, comprising the pattern in an enlarged form

optical means 26 for the collection and for the transmission to the mask24, of the part of the radiation supplied through the back face of thesolid target 28 which the source comprises, the mask 24 supplying animage of this pattern in enlarged form, and

optical means 29 for the reduction of this image and for the projectionof the reduced image onto the layer 20 of photo-sensitive resin.

The target is, for example, made of a material such as silver, copper,tin, samarium or rhenium and has a small thickness (for example, of theorder of 1 μm).

To generate the EUV radiation intended to be used to expose the layer ofphoto-sensitive resin, a pulsed beam 34 emitted by a pulsed laser 35 isfocused on a first face 30 of the target, called the “front face”, usingoptical focusing means 32. The target 28 then emits anisotropic EUVradiation 36 from its back face 37 which is opposite the front face 30.

It should be made clear that the source 22, the optical means 26 forcollection and transmission, the collector 26, the mask 24, the opticalmeans 29 and the support 16 carrying the substrate 20 are placed withinan enclosure (not shown) where low pressure is established. The laserbeam is sent into this enclosure through a suitable window (not shown).

In the example in FIG. 3, the optical means of collection 26 consist ofan optical collector which is arranged facing the back face 37 of thetarget 28, provided to collect the EUV radiation emitted in anisotropicfashion through this back face, to shape this radiation and send ittowards the mask 24.

In the device in FIG. 3, it is not therefore necessary to provideadditional optical means between the collector 26 and the mask 34, whichsimplifies the optical means of the lithography device.

It may be seen that the target of small thickness 28 is fixed by itsfront face 30 to a support 38 provided with an opening 40 that allowsthe passage of the focused laser beam 34 so that it reaches this frontface.

In practice, given that a laser pulse locally destroys the target ofsmall thickness, it is not possible to send the laser beam twice to thesame place on the target. This is why the support 38 is provided withdisplacement means (not shown in FIG. 3) that enable one to exposesuccessively, different zones of the target to the focused laser beam.

This is diagrammatically illustrated in FIG. 4 where one can see a solidtarget 42 of small thickness (for example 1 μm) in the form of a stripfixed to a flexible support 44 which is, for example made of a plasticmaterial and is provided with a longitudinal opening 46 to permitpassage of the focused beam 34.

The target-support assembly forms a composite flexible strip which isunwound from a first spool 48 and is wound onto a second spool 50capable of being rotated by suitable means (not shown). This permitsdisplacement of the target opposite the focused laser beam the pulses ofwhich successively reach different zones of the target. This can then beconsidered as several targets assembled one to another.

In a variant (not shown) it is possible to use a flexible strip ofplastic material as a target support and to fix several targets ontothis support at regular intervals, an opening then being provided in thesupport opposite each target to allow passage of the focused laser beam.

Preferably, instead of a strip of plastic material, a strip 52 (FIG. 5)for example, of copper, silver, tin, samarium or rhenium is used, thisstrip being capable of absorbing the radiation emitted through the frontface of the target 42 under the impact of the focused beam 34 and ofre-emitting this radiation in the direction of this target (which ismobile with the strip 52). This strip 52 has, for example, a thicknessof the order of from 5 μm to 10 μm.

The longitudinal opening that permits passage of the laser beam 34 whichis focused on the target can be defined by two sidewalls 54 and 56approximately parallel to one another and substantially perpendicular tothe target as can be seen in FIG. 5.

However, for better absorption of the radiation emitted through thefront face of the target and better re-emission of this radiationtowards the target, it is preferable that the two sidewalls defining theopening become further apart from one another as they go towards thetarget, as may be seen in FIG. 6 where the two sidewalls have referencenumbers 55 and 57.

In another example, diagrammatically shown in FIG. 7, the target 42 isfixed to a movable support 44 of the kind described when referring toFIG. 4. Furthermore, in the example in FIG. 7, the source of EUVradiation comprises a component 58, fixed in relation to the focusedlaser beam 34 and arranged opposite the front face of the target.

This component includes an opening that permits the passage of the laserbeam that is focused onto this front face of the target and the openingwith which this component is provided, is flared outwards in thedirection of the target and therefore comprises two sidewalls 60 and 62which are inclined with respect to this target and become further apartfrom one another in the direction of the target.

The radiation 64 emitted through the front face of the target 42 is thenabsorbed by these sidewalls 60 and 62 and is re-emitted in the directionof the front face of the target.

The EUV radiation 36 emitted through the rear face of the target istherefore more intense.

Of course, a source of X-rays is known from an article by H. Hirose etal., Prog. Crystal Growth and Charact., Vol. 33, 1996, pp. 277-280, thatuses an emission of X-rays through the back face of a target formed by asheet of aluminum with a thickness of 7 μm, the front face of which isirradiated by a laser beam, with a power density of 3×10¹³ W/cm².

However, it should be noted that the source used in the presentinvention preferably comprises a target of small thickness, thisthickness being within the range extending from about 0.05 μm to about 5μm, this target preferably being made of a material of atomic number Zwhich is greater than that of aluminum since Z is preferably greaterthan or equal to 28 (and lower than or equal to 92).

It may be made clear that the preferred material to form the target usedin the present invention is tin, the atomic number Z of which has thevalue 50.

Furthermore, in the invention, one can use a target of very smallthickness, less than or equal to 1 μm, formed on a substrate of plasticmaterial (for example a substrate of CH₂ (polyethylene) 1 μm thick), theback face of this target (preferably made of tin)—the face which emitsthe EUV radiation used—rests on this substrate. It is also possible toform, on the front face of this target, a layer of gold the thickness ofwhich is less than 1000 Å (that is to say 100 nm).

Returning to the article mentioned above, it should be noted that thetarget of aluminum 7 μm thick cannot be considered for an emissionthrough its back face when its front face is irradiated by laserradiation of a maximum power density less than the 3×10¹³ W/cm²mentioned in the article and that, in particular, in the field ofmicro-lithography, the maximum power density considered above, is, forexample, close to 10¹² W/cm².

One should also note the following:

When the laser interaction takes place on a material of low atomicnumber Z, such as aluminum (Z=13), the transport of the laser energyabsorbed in the corona (the side where the laser interacts front face)towards the dense and cold zones (that is to say towards the back face)occurs by electronic thermal conduction. Even in the case where thetarget is relatively thick such as that proposed in the articlementioned above, obtaining anisotropic emission at the back face is notat all guaranteed.

Contrary to this, in the case of a material with a high Z, it is theradiation conduction which “controls” the turn-on of the interior andthe rear of the target. The anisotropy which creates the interest in thetarget used in this invention is directly linked to the emergence ofthis radiation wave at the back face and therefore to the choice ofthickness, an optimized value for which will be given below.

The characteristic temperature and electronic density profiles in thetarget irradiated by laser are in addition very different depending onwhether the material is of low or high atomic number and also dependingon the target thickness used.

An analytical model enables one to find the optimum thickness E₀ whichpermits optimization of the rate of conversion X at the back face. E₀ islinked to the atomic number Z of the material of the target, to theatomic mass A of this material, to the temperature T (in ° K.) in themedium (itself linked to the laser flux absorbed φ_(a) expressed inW/cm²), to the wavelength λ of the laser (in μm), to the pulse durationDt (in seconds) and to the density ρ (g/cm³), by the following formula:

E ₀ (in cm)=26.22(A/Z)^(0.5) ×T ^(0.5) ×D _(t)/α

with α=ρ×λ²×(1+0.946(A/Z)^(0.5))

The temperature (in ° K.) is proportional to φ_(a) ^(2/3) and toλ^(4/3).

For a low available laser energy (lower than 1J), which is generallynecessary within the context of lithography, since a very highthroughput (greater than 1 kHz) is called for to produce adequate valuesat the photo-sensitive resin (and thereby guarantee that the exposurethreshold is reached) and a given size (for example close to 300 μmdiameter) for the emissive zone (imposed by optimum coupling with theoptical system used), the laser flux falling on the target is low. In ananosecond regime, it does not exceed 10¹² W/cm² at 1.06 μm.Furthermore, at the present time, it is not considered practical tomanufacture lasers at these throughputs working with a series of pulsesof 100 ps.

Under these conditions, the model above gives a value of 30 eV, as amedium temperature that it is possible to achieve if all the energy isabsorbed.

Under these conditions, for aluminum, the optimum thickness whichoptimizes the rate of conversion X at the back face is 0.15 μm, which isvery far from the conditions given in the article mentioned above.Furthermore, with a material such as aluminum, of low atomic number, theradiation emitted through the back face of the target does not, apriori, have any angularity: it is substantially isotropic; the frontface and the back face can therefore be considered to be equivalent.

With the example of gold, still under the same conditions, thisthickness is less than 0.1 μm.

To return to the example given above of a target made of tin, formed ona substrate made of CH₂ (polyethylene), the following details are given:both the polyethylene which can be put on the back face of a thin sheetof tin and the gold which can be put on the front face of this sheet areused to limit the expansion of the emitting material constituted by thetin before it is heated up by the radiation wave, this being in such away that the photons are driven into the zone of the target which is ofinterest. The polyethylene at the back face, which is slightly heated,is transparent to the radiation and also limits expansion and thereforeto a small extent the emission of material debris.

Before describing an example of a multi-layered mirror that may be usedin the invention, we shall return to a multi-layered mirror, intended toreflect radiation from the extreme ultraviolet range, making referenceto FIG. 8.

This known multi-layered mirror comprises a substrate 64 for example,made of silicon and, on this substrate 64, a stack of layers 66 of afirst material and of layers 68 of a second material which alternatewith the layers of the first material.

This first material (for example molybdenum) has an atomic numbergreater than that of the second material (for example, silicon).

The first and second layers work together to reflect the radiation fromthe extreme ultraviolet domain within a wavelength range centered on aspecified wavelength.

The stack has a free surface 70 on which the radiation 72 that onewishes to reflect arrives.

In this known multi-layered mirror, the thickness d of pairs of adjacentlayers of the stack is constant. This thickness d is called theinter-reticular distance.

The angle of attack of radiation 72 that one wishes to reflect isdesignated θ. This angle is the complement of the angle of incidence ofthis radiation. In addition the wavelength of the reflected radiation isdesignated λ and k is the order of reflection.

Alternating layers of the first material, or heavy material, and layersof the second material, or light material, induces a periodic variationof the optical index in relation to the thickness. This variationpermits selective reflection of the incident radiation.

In effect, if an electromagnetic wave strikes a large number ofequidistant reflecting layers, the interferences are everywheredestructive in the direction of the reflected waves except where thepath difference is equal to a whole number of wavelengths.

This selective reflection phenomenon can be described by a law analogousto Bragg's Law

2d×sin θ=k×λ

In FIG. 9, curve I has been drawn which represents variations inreflector power P (in arbitrary units) of a multi-layered mirror of thekind shown in FIG. 8, as a function of the energy En (in eV) of theincident radiation, for specified values of k and θ. The breadth atmid-height of this curve I is about 6 eV.

The multi-layered mirror in FIG. 8 is therefore a regular structurewhich has a narrow bandwidth.

In this invention, it is desirable to use multi-layered mirrors with abroad bandwidth so as to collect photon fluxes which are as great aspossible.

In order to provide this increase in bandwidth, conforming to theinvention, the inter-reticular distance d is gradually modified when theradiation penetrates into the multi-layer.

Therefore, one must choose the nature and the successive thicknesses ofthe deposited layers in order to adapt the structure of themulti-layered mirror.

Optimization of this multi-layered mirror (with regard to the nature andthe thicknesses of the deposited layers) is carried out using arecursive calculation code for transport of the beam of EUV radiationwithin the stack of layers.

FIG. 10 is a diagrammatic longitudinal section view of a particularembodiment of multi-layered mirrors that can be used in the invention.

The multi-layered mirror in FIG. 10 comprises a substrate 74 and, onthis substrate 74, a stack of layers 76 of a first material and layers78 of a second material which alternates with the layers of the firstmaterial, this first material, or heavy material, having an atomicnumber greater than that of the second material, or light material.

The first and second layers work together in order to reflect the EUVradiation within a wavelength range centered on a specified wavelength.

The free surface 80 of the stack onto which the EUV radiation 82 to bereflected arrives, may also be seen in FIG. 10.

In contrast to the known multi-layered mirror in FIG. 8, in themulti-layered mirror in FIG. 10, the thickness of the pairs of adjacentlayers of the stack is an increasing function of the depth in the stack,this depth being counted from the free surface 80 of the stack.

In the example represented in FIG. 10, the first and second materialsare respectively molybdenum and silicon and the substrate 74 is made ofsilicon. However, beryllium may also be used as the second material andthe substrate 74 may be made of germanium.

In the example in FIG. 10, the stack is made up of several groups eachcomprising a plurality of double layers (a layer of first material andan adjacent layer of second material), for example seven double layersor eight double layers, and the thickness of the groups increases as onepasses from the free surface 80 of the stack to the substrate 74. Theincreases in thickness of the groups form, for example, an arithmeticprogression and, within each group, all the layers have substantiallythe same thickness.

By way of an example, as one passes from the free surface 80 to thesubstrate 74, there are seven pairs having a total thickness E1, thenseven pairs having a total thickness E1+ΔBE, then seven pairs having atotal thickness E1+2ΔE, and so on as far as the substrate 74.

In the multi-layered mirror in FIG. 10, the total thickness of the stackof layers is, for example, 1 μm.

The EUV radiation 82 is reflected on consecutive diopters formed by theheavy material and the light material. If the constructive interferencecondition between the reflected waves on the various diopters is met(2d×sin θ=k×λ), the radiation leaves the multi-layers (Bragg's Law).

The thickness of the substrate 74 depends on the shape and the degree ofpolishing of this substrate. This thickness of the substrate 74 isbetween 5 mm and 40 mm.

In order to produce a stack of the kind in FIG. 10, all the layers 78and 76 are deposited successively, with the desired thicknesses, on thesubstrate 74, for example using cathodic sputtering.

Of course, the use of multi-layers having a configuration comparable tothat in FIG. 10 is known, but for a totally different range ofwavelengths and for a totally different function: these knownmulti-layered stacks are used as band-pass filters for radiation in thevisible range.

It should be noted that, in the EUV range, the design of multi-layeredmirrors that can be used in the invention is specific, in particularwith regard to the nature, the thicknesses, the densities, the opticalconstants of the materials and the quality of the deposits.

In the lithography device in FIG. 3, the multi-layered mirrors that formthe collector 26, the mask 24 and the optical projection and reductionmeans 29 are multi-layered mirrors of the kind in FIG. 10 and arecapable of reflecting the EUV radiation, the wavelengths of which arecentered on a specified wavelength (for example 12 nm).

In particular, the collector 26 can be formed by the joining of severalelementary collectors which constitute multi-layered mirrors of the kindin FIG. 10.

FIG. 9 shows the curve II that represents variations in the reflectorpower P (in arbitrary units) as a function of the energy En (in eV), fora multi-layered mirror that can be used in the invention, for example ofthe kind in FIG. 10.

The large increase in the mid-height width, which is 9 eV for the curveII, in comparison with a multi-layered mirror of the prior art (curve I)can be seen.

Therefore, in the invention, the band width of the multi-layered mirrorsfor EUV radiation is increased.

An explanation will now be given of a way of minimizing the thermaldeformation undergone by any multi-layered mirror and, in particular amulti-layered mirror that can be used in the invention when thismulti-layered mirror is exposed to intense EUV radiation.

In order to obtain such a mirror, about a hundred pairs of layers ofsuitable thickness (layers of heavy material alternating with layers oflight material) are deposited for example, on a silicon substrate,optically polished to the desired shape, so as to obtain a totalthickness of layers of the order of 1 μm. This thickness is thereforenegligible in comparison with that of the substrate (for example, a fewmillimeters) which provides the shape of the multi-layered mirror.

The deformation of a flat mirror subjected to a thermal flux density onits front face is of a geometric nature. This deformation has twocomponents.

The first component is parallel to the surface of the plate forming themirror. This first component leads to general spherical curvaturethrough a bilaminar effect and results from the temperature differencebetween the front face and the back face of the mirror.

The second component is perpendicular to the surface of the mirror andcauses local deformation, namely a local increase in the thickness ofthe mirror. It is due to the lack of homogeneity of the density of thethermal flux to which the mirror is subject.

The general curvature (bilaminar effect) is diagrammatically illustratedin FIG. 11. The incident radiation 84 on the multi-layered mirror can beseen.

The temperature difference ΔT_(s) between the front face and the backface of the mirror causes general spherical curvature with an associatedmaximum slope Δp.

For a non-cooled mirror, the edges of which are free, this slope isexpressed by the following equation in which φ is the thermal fluxdensity (in W/mm²), α the thermal coefficient of expansion of themirror, k the thermal conductivity of the mirror, C a constant of value1 for a spherical curvature and ½ in the case of a cylindrical curvatureand l_(i), half of the length of the mirror:

Δp=C×(α/k)×φ×l _(i)

This slope associated with the general curvature varies with theincident flux in a linear fashion. It becomes greater as the ratio α/kbecomes greater and when the dimensions of the beam on the mirror arelarge.

The slope Δp is independent of the thickness of the mirror and of theangle of attack of the radiation on this mirror. The radius of curvatureassociated with the deformation of the mirror does not depend on thedimensions of this mirror. This radius of curvature R is expressed bythe equation:

R=φ ⁻¹×(k/α).

Th localized deformation which is diagrammatically illustrated in FIG.12 will now be considered. This local deformation is due to theexpansion of the mirror perpendicular to its surface. It is due to thelack of homogeneity of the flux incident to the mirror. This lack ofhomogeneity is induced by the angular divergence of the beam 84 whichmay, for example, follow a Gaussian distribution law.

The maximum slope Δh associated with this localized deformation isexpressed by the following equation:

Δh=2×(α/k)×(e ² /Li)×φ₀.

In this equation, φ₀ is the flux density at the center of the print ofthe beam on the multi-layered mirror, e the thickness of this mirror, αthe coefficient of thermal expansion, k the thermal conductivity of themirror and L_(l) the width, at mid-height, of the print of the beam onthe mirror.

The slope Δh varies in a linear way with the incident flux. It becomesgreater as the ratio α/k becomes greater and as when the impact of thebeam on the mirror is small. This slope varies with the square of thethickness of the mirror.

In order to reduce the effects of these mechanical deformations, it isnecessary that the print of the beam on the multi-layered mirror shouldhave large dimensions, in order to “spread” the thermal flux density,and requires a mirror of small thickness, that is not very absorbentwith respect to the radiation and which has a low α/k ratio.

The print of the beam on the multi-layered mirror depends on the angleof attack chosen for the reflection. This angle of attack is close to90°, which minimizes the print of the beam.

The choices of the kind of mirror and its thickness depend on thepolishing techniques that permit one to obtain the desired shape andsurface roughness.

The thermal conductivity k and the coefficient of thermal expansion αvary as a function of temperature. As shown in FIG. 13, in the case ofsilicon, one can benefit from the very low coefficient of expansion α ofthis material, associated with high thermal conductivity k when thetemperature is close to 125 K.

In FIG. 13, one can see curve II which represents variations in thecoefficient of thermal expansion α of silicon (in 10⁻⁶ K⁻¹) as afunction of the temperature T (in K) and curve I which representsvariations in the thermal conductivity k (in W/m.K) as a function of thetemperature T (in K).

In this case, the ratio α/k tends towards 0 when the temperature tendstowards zero, which minimizes the mechanical deformations due to thethermal flux. One is referred to FIG. 14 which shows the curve whichrepresents variations in α/k (in 10⁻⁶ m/W) as a function of thetemperature (in K).

Preferably, in the invention therefore, the multi-layered mirrors forexample of the kind in FIG. 10 are cooled to a low temperature close to100 K, in order to minimize the mechanical deformation due to thethermal flux during use of the EUV lithography device, whatever thematerial of the substrate of the mirror (silicon or germanium forexample).

FIG. 15 diagrammatically illustrates this. A multi-layered mirror can beseen, which comprises a stack 88 of alternating layers on a substrate 90and which is cooled. In order to do this, the mirror is placed on asupport 92 within which liquid nitrogen is circulated. As a variant,this support 92 contains a reservoir of liquid nitrogen.

In this way, deformation of the multi-layered mirror is reduced when itreceives a high EUV radiation flux 94.

Returning to FIG. 10. In the example in this FIG. 10, the thickness ofthe pairs of adjacent layers of the stack of the multi-layered mirror isa function that increases with the depth in the stack. Nevertheless amulti-layered mirror that can be used in the invention is obtained inwhich the thickness of the pairs of adjacent layers is a function whichdecreases with the depth in this stack.

What is claimed is:
 1. A lithography device comprising: a support (16)for a sample intended to be exposed in accordance with a specifiedpattern, a mask (24) comprising the specified pattern in an enlargedform, a source (22) of radiation in the extreme ultraviolet range,optical means (26) for collection and for transmission of the radiationto the mask, the latter supplying an image of the pattern in enlargedform, and optical means (29) for reduction of this image and for theprojection of the reduced image onto the sample, the mask, the opticalmeans for the collection and the transmission and the optical means forreduction and projection comprising multi-layered mirrors, eachmulti-layered mirror comprising a substrate (74) and, on this substrate,a stack of layers (76) of a first material and of layers (78) of asecond material which alternate with the layers of the first material,this first material having an atomic number greater than that of thesecond material, the first and second layers co-operating to reflect theextreme ultraviolet radiation, the stack having a free surface (80) ontowhich the radiation to be reflected arrives, this device beingcharacterized in that the source comprises at least one solid target(28), having first and second faces, this target being capable ofemitting the extreme ultraviolet radiation by interAction with a laserbean (34) focused on the first face (30) of the target, this targetbeing capable of emitting, in an anisotropic way, a part (36) of theextreme ultraviolet radiation from the second face (37) of this target,in that the optical means (26) for collection and for transmission areprovided in order to transmit, to the mask (24), the part (36) of theextreme ultraviolet radiation coming from the second face (37) of thetarget of the source and in that the thickness of pairs of adjacentlayers (76, 78), in the stack of layers that each mirror comprises, is amonotonic function of the depth in the stack, this depth being countedfrom the free surface (80) of the stack.
 2. Device according to claim 1,in which the target (28) contains a material which is capable ofemitting the extreme ultraviolet radiation by interAction with the laserbeam and the thickness of the target is within a range extending fromabout 0.05 μm to about 5 μm.
 3. Device according to claim 1, in whichthe target (28) contains a material which is capable of emitting theextreme ultraviolet radiation through interAction with the laser beamand which has an atomic number belonging to the group of atomic numbersranging from 28 to
 92. 4. Device according to claim 1, comprising aplurality of targets (42) which are made integral one with another, thedevice additionally comprising means (48, 50) of displacing thisplurality of targets so that these targets successively receive thelaser beam (34).
 5. Device according to claim 4, additionally comprisingsupport means (38, 44, 52) to which the targets (42) are fixed and whichare capable of allowing the laser beam to pass in the direction of thesetargets, the means of displacement (48, 50) being provided in order todisplace these means of support and hence the targets.
 6. Deviceaccording to claim 5, in which the means of support (52) are capable ofabsorbing radiation emitted by the first face of each target whichreceives the laser beam and of re-emitting this radiation towards thistarget.
 7. Device according to claim 5, in which the means of supportcomprise an opening (40, 46) facing each target, this opening beingdefined by two sidewalls (54, 56), substantially parallel to one anotherand perpendicular to this target.
 8. Device according to claim 5, inwhich the means of support comprise an opening facing each target, thisopening being defined by two sidewalls (55, 57) which become furtherapart from one another as they go towards the target.
 9. Deviceaccording to claim 1, additionally comprising auxiliary fixed means (58)which are capable of allowing the laser beam (34) to pass in thedirection of the target, of absorbing the radiation emitted by the firstface of this target and of re-emitting this radiation towards thistarget.
 10. Device according to claim 1, in which the stack issubdivided into assemblies of at least one pair of first and secondlayers (76, 78) and the thickness of these assemblies is a monotonicfunction of the depth in the stack, this depth being counted from thefree surface (80) of the stack.
 11. Device according to claim 10, inwhich the increases in thickness of these assemblies form an arithmeticprogression.
 12. Device according to claim 10, in which the first andsecond layers (76, 78) of each assembly have approximately the seinethickness.
 13. Device according to claim 1, in which the first andsecond materials are respectively molybdenum and beryllium or molybdenumand silicon.
 14. Device according to claim 1, in which the substrate(74) is made of a material which is chosen from among silicon andgermanium.
 15. Device according to claim 1, in which the thickness ofthe substrate (74) is within the range extending from about 5 mm toabout 40 mm and the thickness of the stack is about 1 μm.
 16. Deviceaccording to claim 1, in which each multi-layered mirror is fitted withmeans (92) of cooling this multi-layered mirror to reduce deformation ofit when it is illuminated by the extreme ultraviolet radiation. 17.Device according to claim 16, in which the cooling means (92) areprovided in order to cool the multi-layered mirror to a temperatureroughly equal to 100 K.
 18. Device according to claim 16, in which themeans (92) of cooling the mirror are means of cooling by liquid helium,Freon, liquid nitrogen or a cooling fluid which is a heat transfer fluidat a low temperature close to 0 K.
 19. Device according to claim 1,wherein the sample comprises a semi-conductor substrate (18) on which alayer (20) of photo-sensitive resin is deposited and which is intendedto be exposed in accordance with the specified pattern.